The win follows the agency's appointment of senior account manager Kerry Hartigan, who handled the account at regional incumbent agency Ekay.
MEC will begin working on the retail park's media planning and buying business from the start of next year, although Hartigan has already moved to the WPP agency's London office.
Opened in 1990, and with more than 300 stores under one roof, the Essex-based destination site is one of the UK’s leading indoor shopping centres.
Separately, after an eight-month wait, MEC has also been declared the winner of Aegon's £3m media account, previously held at Ptarmigan Media. The Dutch insurer first called the review in October last year, after the appointment of its UK director of marketing Steve Clode.